(1) Field of the Invention
This invention relates to a transmission apparatus and, more particularly, to a transmission apparatus for transmitting data in an area and between areas according to a level.
(2) Description of the Related Art
With optical communication networks for realizing large-capacity communication, usually a wide area is dotted with a large number of transmission apparatus. For example, transmission apparatus used on a synchronous optical network (SONET) in North America are located so as to link chief cities in North America. Such transmission apparatus are remotely controlled by, for example, a centralized control center via a network. With SONET, an area in a SONET overhead called data communication channel (DCC) bytes is used for exchanging data between a centralized control center and each transmission apparatus.
FIGS. 22A, 22B, and 22C are views for describing a SONET frame. With SONET, as shown in FIG. 22A, nine pieces of data each including a 3-byte header and 87-byte data are exchanged in a period of 125 μsec. These nine pieces of data are recognized as one piece of data and, as shown in FIG. 22B, an overhead and a payload are recognized. The overhead can be divided into a section overhead (SOH) and a line overhead (LOH). A manager in a centralized control center controls a transmission apparatus in a remote area via a transmission apparatus in the centralized control center. Supervisory control information is exchanged between the transmission apparatus in the centralized control center and the transmission apparatus in the remote area by using an area in the section overhead called the DCC bytes shown in FIG. 22C.
The transmission apparatus in the centralized control center and the transmission apparatus in the remote area communicate with each other by designating a protocol stack corresponding to the OSI reference model in this DCC-byte area. FIG. 23 shows an example of a protocol stack for a centralized control center and a transmission apparatus. As shown in FIG. 23, a communication protocol for a centralized control center and a transmission apparatus can be divided into the layers in the OSI reference model. A manager in the centralized control center who manages the transmission apparatus controls the transmission apparatus by sending and receiving a command language called TL1 at the application layer.
The intermediate system to intermediate system (IS-IS) protocol is at the network layer in the OSI reference model. The IP protocol is famous as a protocol at the network layer. With the IP protocol, an IP address is used for specifying a destination and selecting a route to the destination. With the IS-IS protocol, on the other hand, a network service access point (NSAP) address is used for specifying a destination and selecting a route to the destination.
FIG. 24 is a view for describing an NSAP address. As shown in FIG. 24, an NSAP address can be divided into an area address, a system ID (SYSID), and SEL. If the area addresses of transmission apparatus are the same, then they are referred to as “being in the same area”. If these transmission apparatus exist in the same area, then they directly exchange data. This routing is referred to as level-one routing and an apparatus which handles level-one routing is referred to as a level-one apparatus.
On the other hand, transmission apparatus the area addresses of which are different communicate with each other via an apparatus referred to as a level-two apparatus. Usually a level-two apparatus belongs to an area and can also handle level-one routing. Accordingly, a level-two apparatus is referred to as a level-one/two apparatus. When a first transmission apparatus, being a level-one apparatus, sends data to a second transmission apparatus which is a level-one apparatus and which is in another area, the first transmission apparatus must request a transmission apparatus which is a level-one/two apparatus and which belongs to the same area to transmit the data.
By the way, the number of transmission apparatus which are level-one apparatus and which can exist in the same area (the number of transmission apparatus which can handle level-one routing) depends on the size of a routing table each transmission apparatus can hold and therefore on the capacity of a memory included in each transmission apparatus. That is to say, there is a limit to the number of transmission apparatus which can belong to the same level-one routing area. If this limit is reached, a new transmission apparatus cannot be connected to a network. For example, if a new transmission apparatus is connected by mistake, level-one routing is not applied to this new transmission apparatus. To avoid this problem, a second area is newly formed and this transmission apparatus is connected in the new area as a level-one apparatus.
FIGS. 25A and 25B are views for describing an example of the case where a second area is newly formed and where a new level-one apparatus is connected. In FIGS. 25A and 25B, network elements (NEs) 101, 102, and 111, being transmission apparatus, and a network 103 are shown. It is assumed that the network 103 includes a plurality of network elements.
In an existing area 100 shown in FIG. 25A, the number of NEs connected has reached a limit. It is assumed that a new NE 111 is connected in the existing area 100. In this case, as shown in FIG. 25B, the NEs 101 and 102 are changed to level-one/two apparatus first. An area address of the NE 102 is then changed to a new area address. The new area address is set for the NE 111 and the NE 111 is connected to the NE 102. As stated above, if the number of NEs in the routing area has reached the limit, a second area is newly formed and the new NE 111 is connected in the second area. By doing so, a new NE can be connected.
To connect a second NE in a new area after area division, a new area address should be set in advance for the NE before connection.
FIG. 26 is a view for describing the case where an NE is connected in a new area. In FIG. 26, an existing area 130 and a new area 131 separated from the existing area 130 are shown. The existing area 130 includes an NE 141 and a network 142. The new area 131 includes NEs 143 and 144. To connect a new NE 145 in the new area 131 after the area division, an area address of the new area 131 must be set in advance for the NE 145.
If traffic concentrates at part of NEs because of, for example, an improper network design, throughput deteriorates. For example, if part of connection forms a ring, such a state may arise at an NE which is at a position, such as a hub, in an area. In such a case, it is necessary to review the network design and make an adjustment by, for example, area division. By doing so, proper traffic can be realized. In many cases, such deterioration in throughput due to, for example, a network design goes unnoticed until a concrete phenomenon (unstable communication, for example) occurs. When a change in area is made, a communication path which has already connected must be disconnected once because a destination is changed to another area. Connection must be made again to a new area address. Accordingly, while area division is being made, NE supervision must temporarily be stopped.
FIG. 27 is a view for describing area division. In FIG. 27, NEs 151 and 152 and networks 153 and 154 are shown. It is assumed that an area is divided at a division point C shown in FIG. 27 because of traffic concentration. In this case, the NE 151 in an existing area and the NE 152 which belongs to a new area as a result of the area division adjacent to the area division point C are changed to level-one/two apparatus. An area address of the NE 152 is changed to a new area address. The new area address is set for each NE in the new area. As a result, data transfer can be performed between the existing are and the new area.
A routing method and a hierarchical communication network which significantly improve routing for efficiently utilizing a communication network have conventionally been proposed (see, for example, Japanese Patent Laid-Open Publication No. Hei8-32620). In addition, a packet transfer path control apparatus which can secure the scalability and stability of routing handling by linking routers in parallel without the current link state routing handling section being changed has been proposed (see, for example, Japanese Patent Laid-Open Publication No. 2002-57697).
With the conventional area division, however, a maintainer recognizes when to make area division, determines how to divide an area, and manually performs setting and the like. This requires much labor.